If you are making yeast starters where the necessary oxygen for the yeast reproduction is allready done by the time the slurry is pitched, is it a safe assumption that oxygenation of the wort itself is un-necessary? Just a thought..

I think the idea is to end up with 2 replication phases. One in the starter and one in the beer. So if one brand new vial is adequate for 5 gal of 1.040 wort, that is because the yeast supplier is counting on the yeast replicating up to the proper yeast count when it is pitched into the wort, and it is best if they get some o2 for that. If you are fermenting a 1.055 wort, you need to pitch with a greater number of cells, so we increase that with a starter. I would recommend aerating the wort.

No, The yeast will still need oxygen to ferment the wort. If not you are limiting the ability of the yeast to do their thing.

Fermentation is an anaerobic process. That is why we don't add o2 after the yeast replication phase. They do need o2 to make healthy children and healthy cell walls, but once this first phase is done, they do not use air to ferment.

I think you are asking, "If I make a starter big enough to produce enough yeast to ferment my whole 5 gallon batch so the yeast don't have to reproduce after I pitch them, then I don't need to add oxygen, right?"

Technically that would be correct. Yeast only need oxygen during the reproductive phase. However, we typically want the yeast to reproduce in our fermentations. This is a quote from Mrmalty.com:

"Yeast do need some growth to result in the right kind of ester profile"

Thanks for the insight...what I'm taking away here (correct me if I'm wrong) is that if a big enough starter is prepaired, oxygenation of the wort is not necessary but if its only a simple starter to get the yeast active, additional o2 is needed for the cells to reproduce.

I think you are asking, "If I make a starter big enough to produce enough yeast to ferment my whole 5 gallon batch so the yeast don't have to reproduce after I pitch them, then I don't need to add oxygen, right?"

Right, but this would basically be a 5 gallon batch pitched to the proper amount to get growth and at the height of fermentation. You would then have to filter all of those cells out and pitch them into an o2 free wort. The problem is, they will still most likely use other compounds to replicate in that environment. But can stress out the yeast and cause them to create more mutants.

According to White's book on Yeast, though yeast do not need to take in oxygen during fermentation, this is because during the aerobic phase they take in enough oxygen to carry them through the fermentation phase. Without this they won't be able to finish the fermentation. The oxygen added at the beginning of the starter has been used for reproduction and to carry it through fermenting the starter. Even if there were enough yeast to ferment the batch they would still need a fresh infusion of oxygen to carry them through the process.

This is the reason some people aerate again (18 hours or so into the process) when fermenting really big batches.

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Thanks for the insight...what I'm taking away here (correct me if I'm wrong) is that if a big enough starter is prepaired, oxygenation of the wort is not necessary but if its only a simple starter to get the yeast active, additional o2 is needed for the cells to reproduce.

Is this a theoretical question, or a practical one? If you have the ability to make a proper starter and the ability to aerate the wort, it is a lot more practical.

I do not use pure o2, although that would be the best option from a control standpoint. I use a pump with a filter and air stone. I also do not airlock my Carboys during growth phase, just a piece of sanitized tin foil.

You can get pretty good aeration from just swirling the thing every few minutes for 15-30 minutes. Especially if you don't airlock it for the first 24 hours or so.